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Primers and Sealers

Jun 21, 2016 | Expert's Corner | 0 comments

I’ve said this many times, but it deserves repeating because so much confusion still exists. Primers and sealers perform entirely different functions.

Primers are necessary for paint because paint won’t bond well to rough, porous wood. Paint contains a high percentage of pigment (in order to hide well) and only enough binder (the same as finish) to glue the pigment particles to each other and to an underlying smooth surface.

Because even well sanded wood is still porous, and thus not smooth, more binder is required to achieve a good bond. Paint primer contains a higher binder-to-pigment ratio than does paint, so primer creates a smooth surface for subsequent coats of paint to bond to. (Primer also doesn’t hide as well as paint.)

In contrast, finishes always bond well to wood because they are essentially pure binder. Sanding sealer, shellac and vinyl sealer, each of which is often recommended for “sealing,” do nothing to improve bonding on clean wood. On the contrary, sanding sealers and non-dewaxed shellac weaken bonding, because the finish doesn’t bond as well to them as to the wood.

Think of sanding sealer as a production tool. It sands easily and doesn’t gum up sandpaper. Use sanding sealer when finishing large projects.

Think of shellac as a problem solver, usually when refinishing. Shellac blocks silicone, which is often introduced into the wood by furniture polish; silicone in the wood causes most finishes to fish eye. Shellac also blocks off bad odors—for example, from animal urine or smoke damage. For new wood, shellac seals off resinous knots in softwoods such as pine so another finish applied on top dries normally.

Think of vinyl sealer as a first coat under a catalyzed finish to reduce cost and embed coloring steps without a time-limit window.